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Rings and asteroids may explain ‘alien megastructure’ star

Planet-trailing asteroids may cause part of the dimming of Tabby’s star

NASA/JPL-Caltech

By Shannon Hall

It may not be aliens after all. A fresh model that might explain the so-called “alien megastructure” star evokes only natural phenomena, such as a ringed planet that puts Saturn to shame and clouds of comets that contain more mass than Jupiter.

That included Fernando Ballesteros at the University of Valencia in Spain and his colleagues. Although they classify themselves as extragalactic astronomers, they couldn’t help but focus their attention a little closer to home when Tabby’s star became popular. The mystery has been the topic of debate, and now Ballesteros has come up with a solution.

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The team believes a dip in 2011, which reduced the brightness of Tabby’s star by up to 15 per cent, can be explained by a massive ringed planet five times the size of Jupiter transiting in front of it. Rings would help clear up why the dip was asymmetric: if they were tilted, the leading edge might block the star’s light and the trailing edge might not.

A later dip in brightness, which actually consisted of a series of dips in 2013, can be explained by a trailing swarm of asteroids – much like the Trojan asteroids that share an orbit with Jupiter.

And the latest dip, observed the week before last, can be explained by the planet passing behind the star. Surprisingly, this would also create a slight dip, because any light reflecting off the planet – albeit faint – would now be hidden from view.

To boot, the model predicts that another dip will appear in 2021 thanks to a second cloud of asteroids – which usually come in pairs. This forecast has excited the wider community because it will indicate that astronomers are on the right track if the star dims that year.

Problem of scale

But the model isn’t perfect. “There are lots of problems, but at least they are only ‘scale’ problems,” says co-author Alberto Fernández-Soto at the University of Cantabria in Spain. In other words, we have seen these things before, just on much smaller scales, he says.

The planet, for example, would be so large that it could be a small star. And the swarm of asteroids would need to contain as much mass as Jupiter, though it’s not clear whether that is physically possible.

David Kipping at Columbia University in New York and Jason Wright at Pennsylvania State University agree that a strength of the model is that it only uses conventional astrophysics – but scaling is an issue.

“It’s great that we’re getting creative and maybe some parts of this theory might comprise the final answer, but I’m fairly sceptical this is the solution,” says Kipping.

Fernández-Soto also isn’t certain whether this is the right explanation, but is sure that the star’s activity will not be explained with run-of-the-mill objects — at least not in their standard sizes. “Whatever the solution to the riddle, it’s going to be exciting,” he says.